User interface widgets that are part of a web browser's content (such as web forms) are actually handled by a separate toolbox --- rather than XP Toolkit --- and should be assigned to Layout: Form Controls.

STR
Put the contact window in the bottom right corner of the screen, and hover over contact.
The tooltip only shows once its initial size doesn't fit on the screen, vertically at least. (Once it's shown, it can grow successfully.)

(In reply to aleth [:aleth] from comment #0)
> The tooltip only shows once its initial size doesn't fit on the screen,
> vertically at least. (Once it's shown, it can grow successfully.)
Sorry, a typo: it should be "The tooltip doesn't show if its initial vertical size doesn't fit on the screen."

It's possible that the change has somehow affected how these tooltips are shown on OSX. The change has been in the mozilla codebase for around 6 weeks though and does seem to function correctly in Firefox when tooltips are position up against the bottom of the screen. Can you point me to the Instabird codebase so I can take a look?